ALK1 opposes ALK5/Smad3 signaling and expression of extracellular matrix components in human chondrocytes

J Bone Miner Res. 2008 Jun;23(6):896-906. doi: 10.1359/jbmr.080209.


Introduction: TGF-beta is a multifunctional regulator of chondrocyte proliferation, differentiation, and extracellular matrix production. Dysregulation of TGF-beta action has been implicated in cartilage diseases such as osteoarthritis. TGF-beta signaling is transduced through a pair of transmembrane serine/threonine kinases, known as the type I (ALK5) and type II receptors. However, recent studies on endothelial cells have identified ALK1 as a second type I TGF-beta receptor and have shown that ALK1 and ALK5 have opposing functions in these cells. Here we examined ALK1 expression and its regulation of TGF-beta signaling and responses in human chondrocytes.

Materials and methods: ALK1 expression in human chondrocytes was examined by RT-PCR and Western blot. The ability of ALK1 to form complexes with other TGF-beta receptors was determined by affinity labeling/immunoprecipitation and by immunoprecipitation followed by Western blot. The effect of ALK1 on TGF-beta1-induced signaling and responses was determined by varying ALK1 expression levels and measuring transcriptional activity using promoter/luciferase assays, Smad1/5 and Smad3 phosphorylation, and expression of type II collagen, PAI-1, and fibronectin.

Results: Our results indicate that ALK1 is expressed in human chondrocytes and that it is a component of the TGF-beta receptor system, associating with ALK5, type II TGF-beta receptor, endoglin, and betaglycan. Furthermore, we show that both ALK1 and ALK5 are needed for TGF-beta-induced phosphorylation of intracellular mediators Smad1/5, whereas only ALK5 is essential for TGF-beta1-induced phosphorylation of Smad3. In addition, our results show that ALK1 inhibits, whereas ALK5 potentiates, TGF-beta-induced Smad3-driven transcriptional activity and the expression of PAI-1, fibronectin, and type II collagen in chondrocytes.

Conclusions: Our results suggest that ALK1 and ALK5 display opposing functions in human chondrocytes, implicating an essential role for ALK1 in the regulation of TGF-beta signaling and function in these cells.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Activin Receptors, Type II / genetics
  • Activin Receptors, Type II / metabolism*
  • Aged
  • Cells, Cultured
  • Chondrocytes / drug effects
  • Chondrocytes / metabolism*
  • Extracellular Matrix / metabolism*
  • Gene Expression Regulation* / drug effects
  • Humans
  • Male
  • Middle Aged
  • Phosphorylation / drug effects
  • Protein-Serine-Threonine Kinases / genetics
  • Protein-Serine-Threonine Kinases / metabolism*
  • Receptor, Transforming Growth Factor-beta Type I
  • Receptors, Transforming Growth Factor beta / genetics
  • Receptors, Transforming Growth Factor beta / metabolism*
  • Signal Transduction*
  • Smad1 Protein / metabolism
  • Smad3 Protein / metabolism*
  • Smad5 Protein / metabolism
  • Transcription, Genetic / drug effects
  • Transcription, Genetic / genetics
  • Transforming Growth Factor beta1 / pharmacology


  • Receptors, Transforming Growth Factor beta
  • SMAD1 protein, human
  • SMAD3 protein, human
  • SMAD5 protein, human
  • Smad1 Protein
  • Smad3 Protein
  • Smad5 Protein
  • Transforming Growth Factor beta1
  • Protein-Serine-Threonine Kinases
  • ACVRL1 protein, human
  • Activin Receptors, Type II
  • Receptor, Transforming Growth Factor-beta Type I
  • TGFBR1 protein, human